This invention relates generally to systems for testing integrated circuits, and more particularly to the interface between an integrated circuit wafer or chip and a load board.
Conventionally, an integrated circuit test system is connected to the device under test ("DUT") through a load board, which is connected by coaxial cables to a probe card, which is connected to the DUT itself. The probe card is conventionally a printed circuit board with connections for the coaxial cables, fine needles for making contact with the connection points on the DUT, and printed traces on the printed circuit board connecting each cable to a needle.
The quality of test measurements is affected by the nature of the electrical connections, or interface, between the DUT and the test system. It has long been known that long coaxial cables impair the testing of signals, but there has been little success in correcting the problem, despite numerous efforts directed to solving the problem. The bandwidth of coaxial cable is limited. This bandwidth limitation is caused by a high-frequency physical effect called the "internal impedance" of the cable. This effect can cause signal distortion in the cable. Cable-induced signal distortions include mismatch errors between the probe card and cable and between the load board and cable. Cable-induced effects also include power loss and impedance mismatch errors within the cables, dispersion of the transmitted signal, crosstalk, ground loops, and phase distortion.
Nevertheless, flexible and long cables are desired to facilitate movement of the probe card, and to separate spatially the probe card from the load board and the typically large test system. Because the number of channels of an integrated circuit to be tested can number in the hundreds, the bundle of cables can become unwieldy, reducing flexibility and access to the DUT.